Micro-magnetocaloric device

ABSTRACT

A magnetocaloric device ( 1 ), comprises:
         at least one magnetocaloric material ( 5 ) embedded between two heat transfer structures (TD hot , TD cold );   at least one electric source for generating a magnetic field; and   at least one hydraulic circuit in which the working fluid flows in a constant direction and which comprises at least one propulsion means ( 6 ) for the working fluid, wherein the heat transfer structures (TD hot , TD cold ) are adapted to control the transfer or transport of heat between the magnetocaloric material ( 5 ) and the working fluid.

BACKGROUND OF THE INVENTION

The present invention relates to magnetocaloric devices for use inrefrigeration, heat pump, or power generation applications.

FIG. 1 shows a schematic illustration of a generic electric resistivemagnet M. Therein, the thickness of the gap between the poles of themagnetic field is denoted with d. Now, for a fixed width and length ofthe gap, and for the fixed desired magnetic flux density of the gap onemay calculate that the mass and the volume of the magnetic fieldincreases with the order 3 with the increase of the thickness d.

Based on FIG. 2, it can thus be concluded that the thickness of the gapshould be as small as possible, while keeping minimum possible powerinput (since this defines the energy efficiency of the electricresistive magnetic field source). Therefore, it is better to performseveral miniature magnetic field sources compared to a single magneticfield source with the larger thickness of the gap and with theoccupation of the same volume. Of course, the miniaturization of thethickness of the magnetic field gap, e.g. below the range 1 to 5 mm,makes the active magnetic regenerator (AMR) principle or themagnetocaloric porous structure not applicable. Namely the AMR requiresa porous structure of the magnetocaloric material with voids, and themanufacturability of magnetocaloric material (even with predictedimprovements in future) will not be able to provide so fine structures(ordered structures, thickness of the magnetocaloric material below 50microns with acceptable mechanical properties) or packed bed structures.In the latter, the small particles will also create very small voidswith variable hydraulic diameter, which does not result in controlledfluid dynamics and heat transfer. Moreover, the pressure losses in suchstructures are becoming large. This strongly influences the efficiencyof the device, especially if it has to operate with high number ofthermodynamic cycles per unit of time (e.g. above 5 Hz), andsimultaneously provide a predetermined temperature difference betweenthe heat source and the heat sink.

It is thus an object of the present invention to provide an efficientmagnetocaloric device.

SUMMARY

According to a first aspect, a magnetocaloric device is provided,comprising at least one magnetocaloric material embedded between twoheat transfer structures; and at least one electric source forgenerating a magnetic field. With the magnetocaloric material beingembedded between the heat transfer structures, heat transfer from and tothe magnetocaloric material can be achieved in an efficient mannerwithout the need for any working fluid to flow through the porousstructure of the magnetocaloric material. Thus, the device can beminiaturized, since there is no need to provide large pores or voidswithin the magnetocaloric material. The magnetocaloric device of thepresent invention further comprises at least one hydraulic circuit inwhich the working fluid flows in a constant direction and whichcomprises at least one propulsion means for the working fluid. Thus, theworking fluid can constantly flow past the respective heat transferstructures and can thus provide an efficient transport of heat in themagnetocaloric device.

According to embodiments, the heat transfer structures may comprise atleast one thermal switch which is adapted to vary transfer or transportof the heat from the magnetocaloric material to the hydraulic circuit orfrom the hydraulic circuit to the magnetocaloric material. Thus, thedirection of the heat transfer to and from the magnetocaloric materialcan be efficiently controlled using the at least one thermal switch.

According to further embodiments, the heat transfer structures maycomprise at least one multifunctional coating which is adapted to affectthe wetting effect of the working fluid, or/and to affect the thermal orvelocity boundary layer of the working fluid, or/and to affect thechemical protection of the magnetocaloric material, and/or to affect themechanical properties of the magnetocaloric material, and/or to affectthe effective thermal properties of the magnetocaloric material and themultifunctional layer.

According to further embodiments, the heat transfer structures maycomprise at least one means for time varying fluctuation of the heattransfer from the hydraulic circuit to the magnetocaloric material orfor time varying fluctuation of the heat transfer from themagnetocaloric material to the hydraulic circuit, wherein the timevarying fluctuation of the heat transfer may be provided by a timedependent variable fluid flow above and/or below the magnetocaloricmaterial. Thus, the heat transfer between the magnetocaloric materialand the hydraulic circuit may be controlled by the heat transferstructures such that, during a first time period, heat is transferredsubstantially only from the hydraulic circuit to the magnetocaloricmaterial and that, after changing the heat transfer properties of theheat transfer structures, during a second time period, heat istransferred substantially only from the magnetocaloric material to thehydraulic circuit.

According to embodiments, the electric source may comprise electricwindings, a core for the manipulation of the magnetic flux direction,and an electric circuit which enables the regeneration of magneticenergy.

The electric source may be adapted to perform a time dependent variationof increasing or decreasing magnetic field intensity in the rangebetween 0.0001 to 5 seconds. Alternatively or additionally, the electricsource may be adapted to perform a variation of the magnetic fieldintensity in the range between 0 to 40 Tesla. Thus, the electric sourcecan provide e.g. a magnetic field intensity at high frequencies, so thatthe individual thermodynamic cycles of the magnetocaloric device arerepeated at high frequencies in the range of 0.2 Hz-1 kHz.

According to embodiments, the magnetocaloric device may performrefrigeration, heat pumping or power generation by one of a magneticBrayton, magnetic Stirling, magnetic Carnot, or magnetic Ericsson cycle.

According to embodiments, the magnetocaloric device may performrefrigeration, heat pump or power generation by non-conventionalthermodynamic cycles, comprising a combination of magnetic isofieldprocess(es), and/or magnetic isothermal magnetization ordemagnetization, and/or magnetic isentropic or polytropic magnetizationor demagnetization, and/or isomagnetization process(es).

According to embodiments, the hydraulic circuit with the working fluidmay be connected to at least one heat exchanger, such as a heat sourceor/and a heat sink heat exchanger.

According to embodiments, the working fluid propulsion may be created byionic, or magnetohydrodynamic, or magnetocaloric, or magnetorheologic,or ferrofluidic, or electrocaloric, or electrowetting, orelectrophoeretic, or electrokinetic, or electrohydrodynamic principles.

According to embodiments, the working fluid propulsion may be created byat least one piston, or by at least one turbine, or at least onemembrane, or at least one peristaltic mechanism, or at least one ejectorprinciple, or at least one magnetohydrodynamic principle, or at leastone electrohydrodynamic or electrokinetic principle, or at least oneelectrophoretic principle, or at least one electrowetting principle, orat least one ferrohydrodynamic principles, or at least onemagnetorheologic principle.

The magnetocaloric material may be any material which exhibits themagnetocaloric effect in the range of temperature between 0 to 3000 K.

According to embodiments, a plurality of magnetocaloric materials may beused, each of which having a Curie temperature in the range between 0 to3000 K.

According to embodiments, at least one or several magnetic field sourcesand several magnetocaloric materials may be present in order to providean upgrade from the micro-scale to the macroscopic device forrefrigeration or heat pumping, or power generation.

According to embodiments, the device may comprise at least one magneticfield source and a plurality of magnetocaloric materials, wherein eachmagnetocaloric material is embedded between two heat transferstructures, and wherein a common hydraulic circuit is provided such thatthe heat transfer structures are adapted to control the transfer ortransport of heat between each magnetocaloric material and the workingfluid. Thus, a macro-scale magnetocaloric device can be provided byusing a minimum of one, but rather a plurality of micro-scale assembliescomprising respective magnetocaloric material and heat transferstructures. Therein, the micro-scale assemblies may be combined with acommon magnetic field source, such as e.g. a multi-pole magnetic fieldsource, or may be provided with a plurality of magnetic field sources.

Moreover, in a special case, the magnetic field source can represent acombination of the electric coil, combined with permanent magnets.

According to embodiments, a plurality of magnetocaloric devices asdescribed above may be combined to form a cascade system. A cascadesystem comprises a plurality of magnetocaloric devices, where in thecase of refrigeration or heat pumping, the heat sink of the first devicewith lowest temperature represents heat source of the second stage, etc.

According to embodiments, a heat transfer structure comprising at leastone thermal switch may be based on anisotropy of the thermalconductivity of the thermal switch material. Alternatively, the thermalswitch may comprise at least one thermal switch composite material whichexhibits anisotropy of the effective thermal conductivity. In thisparticular case, the anisotropy of the thermal conductivity of thethermal switch changes with external influences (e.g. temperaturechange, magnetic or electric means or others). The aim of such principleis that a thermal switch material or composite allows heat to flow indesired direction at the certain temperature, and prevents or decreasesthe rate of heat to flow in that direction at another temperature.

Alternatively, the thermal switch may be based on mechanical contact byelastomer materials, or liquid crystals, or on ferrofluids, ormagnetorheologic principles, or liquid metals, or electrorheologicprinciples or magnetohydrodynamic principles or electrowetting, orelectrophoeretic or electrokinetic or electrohydrodynamic principles. Inthis particular case, the thermal switch, being in the state of solid,suspension, or liquid, represents a thermal contact, which can bemanipulated by electricity, magnetism, or thermal effects. When heatneed to be removed from the magnetocaloric material, this kind ofthermal switch absorbs heat from the magnetocaloric material (when thisis in magnetized state), and by manipulation of in this paragraphmentioned mechanisms, changes its shape or position in order to performa mechanical contact heat to the heat sink (vie e.g. the extendedsurface). When the magnetocaloric material is demagnetized (e.g. cold),the same thermal switch prevents heat to flow from the heat sink (e.g.via extended surface) to the magnetocaloric material. However, anotherthermal switch at this time provides a mechanical contact between theheat source (via another extended surface) and the magnetocaloricmaterial.

As a further alternative, the thermal switch principle may be based onthermoelectric (Peltier or Seebeck), or thermionic, or spincaloritronic(spin Peltier or spin Seebeck). Also in this case, as an example, themagnetocaloric material may be embedded between two thermal switches ore.g. its upper and lower surface. The thermal switch in this case mayact as a heat pump which is on one side attached to the magnetocaloricmaterial and on the other side attached to the extended surface. Whenthe magnetocaloric material is magnetized, then the e.g. upper thermalswitch is set on (e.g. by setting the electric current flow). Heat isballistically transported from the magnetized magnetocaloric material tothe extended surface. At the same time, the lower thermal switch is setoff, thus preventing heat flow from the magnetocaloric material to theheat source (via the lower extended surface). When the magnetocaloricmaterial is demagnetized, then the upper thermal switch is set off, andthe lower thermal switch is set on, thus heat is pumped from the heatsource (via the lower extended surface), to the magnetocaloric material.

According to a further aspect, a magnetocaloric device is provided,comprising at least one magnetocaloric material embedded between twoheat transfer structures. The device further comprises at least oneelectric source for generating a magnetic field, wherein the electricsource enables regeneration of the magnetic energy, and at least onehydraulic circuit which comprises at least one propulsion means for theworking fluid, wherein the heat transfer structures are adapted tocontrol the transfer or transport of heat between the magnetocaloricmaterial and the working fluid. Thus, during each cycle of themagnetocaloric device according to the further aspect, at least some ofthe magnetic energy produced by the electric source can be regenerated,so that the overall energy efficiency of the device can be increased.

According to embodiments, the electric source may comprise anelectromagnet and an energy collector device. The energy collectordevice may be used for storing at least some of the energy of themagnetic field of the electromagnet, and may further be used to supplythe stored energy to the electromagnet for generating a magnetic fieldin a subsequent operation cycle or operation phase of the device.Therein, the electric source may be adapted to charge the energycollector device when the magnetic field of the electromagnet is turnedoff, and use the charged energy collector device for generation of amagnetic field in the electromagnet when the magnetic field is turnedon.

According to embodiments, the electric source may further comprise afirst switching device for connecting the electromagnet to the energycollector device for charging the energy colletor device and a secondswitching device for connecting the energy collector device to theelectromagnet for turning on the magnetic field in the electromagnet byreleasing the energy stored in the energy collector device to start thecurrent flow through the electromagnet.

According to embodiments, the energy collector device may comprise abattery or a capacitor.

According to embodiments, the magnetic field may be generated by atleast one electric source and at least one permanent magnet material.Thus, a permanent magnet may be used for generation of the magneticfield for magnetizing the magnetocaloric material. Additionally, theelectric circuit may enable regeneration of the magnetic energy in orderto ensure a high energy efficiency of the magnetocaloric device.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to thedrawings, in which like reference numerals denote the same orcorresponding elements, and in which:

FIG. 1 shows a schematic view of an electric resistive magnet;

FIG. 2 shows a diagram of the mass of a electric resistive magnetagainst the magnetic field gap thickness for producing a 1T magneticfield;

FIG. 3 shows a micro-magnetocaloric refrigerator or heat pump accordingto a first embodiment;

FIG. 4 shows a micro-magnetocaloric power generator according to asecond embodiment;

FIG. 5 shows a micro-magnetocaloric refrigerator or heat pump accordingto a third embodiment;

FIG. 6 shows a micro-magnetocaloric power generator according to afourth embodiment;

FIG. 7 shows a micro-magnetocaloric refrigerator or heat pump accordingto a fifth embodiment;

FIG. 8 shows a micro-magnetocaloric power generator according to a sixthembodiment;

FIG. 9 shows a micro-magnetocaloric refrigerator or heat pump accordingto a seventh embodiment;

FIG. 10 shows a micro-magnetocaloric power generator according to aneighth embodiment;

FIG. 11 shows a schematic illustration of heat transfer between themagnetocaloric material and the working fluid;

FIG. 12 shows a multipole magnetic field source with multiple fluidchannels in a cylindrical structure;

FIG. 13 shows an alternative embodiment which employs thermal switches;

FIG. 14 shows a further alternative embodiment which employs amultifunctional surface;

FIG. 15 shows a third alternative embodiment which employs thermalswitches and multiple magnetocaloric materials and a magnetic field withseveral poles;

FIG. 16 shows a fourth alternative embodiment which employsmultifunctional surfaces and multiple magnetocaloric materials and amagnetic field with several poles;

FIG. 17 shows a fifth alternative embodiment which employs a multipolemagnetic field source, multiple magnetocaloric materials and multiplefluid channels in a cylindrical structure;

FIG. 18 shows a schematic of an electric circuit for regenerationmagnetic energy in a magnetic field source;

FIG. 19 shows a schematic operation diagram of a heat engine based onthe magnetocaloric effect and a Carnot thermodynamic cycle;

FIG. 20 shows a schematic of an electric circuit for regenerationmagnetic energy in a magnetic field source;

FIG. 21 shows a schematic of an electric circuit for regenerationmagnetic energy in a magnetic field source;

FIG. 22 shows a diagram of the current flow through the electromagnetwith respect to time in the circuit shown in FIG. 18;

FIG. 23 shows a first embodiment of a magnetic field source structure inwhich a magnetocaloric material is embedded;

FIG. 24 shows a second embodiment of a magnetic field source structurein which a magnetocaloric material is embedded;

FIG. 25 shows a third embodiment of a magnetic field source structure inwhich a magnetocaloric material is embedded;

FIG. 26 shows a fourth embodiment of a magnetic field source structurein which a magnetocaloric material is embedded;

FIG. 27 shows a fifth embodiment of a magnetic field source structure inwhich a magnetocaloric material is embedded; and

FIG. 28 shows a sixth embodiment of a magnetic field source structure inwhich a magnetocaloric material is embedded.

DETAILED DESCRIPTION

In the following description of various embodiments, reference will bemade to the drawings, in which like reference numerals denote the sameor corresponding elements. The drawings are not necessarily to scale.Instead, certain features may be shown exaggerated in scale or in asomewhat simplified or schematic manner, wherein certain conventionalelements may have been left out in the interest of exemplifying theprinciples of the invention rather than cluttering the drawings withdetails that do not contribute to the understanding of these principles.

It should be noted that, unless otherwise stated, different features orelements may be combined with each other whether or not they have beendescribed together as part of the same embodiment below. The combinationof features or elements in the exemplary embodiments are done in orderto facilitate understanding of the invention rather than limit its scopeto a limited set of embodiments, and to the extent that alternativeelements with substantially the same functionality are shown inrespective embodiments, they are intended to be interchangeable, but forthe sake of brevity, no attempt has been made to disclose a completedescription of all possible permutations of features.

Furthermore, those with skill in the art will understand that theinvention may be practiced without many of the details included in thisdetailed description. Conversely, some well-known structures orfunctions may not be shown or described in detail, in order to avoidunnecessarily obscuring the relevant description of the variousimplementations. The terminology used in the description presented belowis intended to be interpreted in its broadest reasonable manner, eventhough it is being used in conjunction with a detailed description ofcertain specific implementations of the invention.

In a first embodiment, as shown in FIG. 3, the micro-magnetocaloricdevice 1 may operate as a refrigerator or as a heat pump, whereinregeneration of the magnetic energy may be provided and heat may beregenerated according to the thermal switch principle in a combinationwith the fluid flow in the hydraulic circuit. The device comprises amagnetic field source 2 with an iron or soft ferromagnetic material core3 and at least one coil winding or other means for electric current flow4. At least one electric circuit (not shown in FIG. 3) may be providedfor the control, regulation of operation and regeneration of themagnetic energy. In the gap between the magnetic poles created by themagnetic field source 2, at least one magnetocaloric material 5 (MCmaterial 5) is provided. As shown in detail in the schematiccross-sectional view on the left hand side of FIG. 3, the MC material 5is positioned between two thermal switch mechanisms denoted by TDcoldand TDhot respectively. To each of these thermal switch mechanisms, arespective extended surface is attached, denoted by EScold and EShot,respectively. Working fluid is pumped by pumping means 6 in a continuouscircuit, as indicated by the arrows, past the respective extendedsurfaces EScold and EShot and through a heat source heat exchanger CHEXand a heat sink heat exchanger HHEX.

The basic operation is the following. When the magnetic field source 2is switched on, the thermal switch TDhot is in operation, thusrepresenting a means for thermal transport from the magnetocaloricmaterial 5 to the extended surface EShot. Through the extended surfaceEShot, the working fluid flows in a direction from the heat source heatexchanger CHEX to the heat sink heat exchanger HHEX, wherein thepropulsion of the working fluid is provided by the pumping means 6 or byany other working fluid propulsion system. In this manner, the workingfluid absorbs heat from the extended surface EShot and rejects it in theHHEX. Before the magnetic field source 2 is switched off, operation ofthe thermal switch mechanism TDhot is deactivated. Thus, thermaltransport between the magnetocaloric material and the extended surfaceEShot can be prevented, wherein the deactivated thermal switch mayrepresent an adiabatic barrier. When the magnetic field source 2 isswitched off, the thermal switch mechanism TDcold is activated, thusrepresenting a means for thermal transport to the magnetocaloricmaterial 5 via the thermal switch from the extended surface EScold.Through the extended surface EScold, the working fluid flows in adirection from the heat sink heat exchanger HHEX to the heat source heatexchanger CHEX, wherein the propulsion of the working fluid is providedby the pumping means 6 or by any other working fluid propulsion system.In this manner, the working fluid transfers heat from the extendedsurface EScold via the thermal switch TDcold to the magnetocaloricmaterial. 5 This cycle is then repeated. The working fluid may flowcontinuously or discontinuously, depending on desired thermodynamiccycle. Furthermore, the working fluid flow, the operation of thermalswitch mechanisms and the operation of the magnetic field source can betuned in a manner that the micro-magnetocaloric device operates withdifferent thermodynamic cycles.

FIG. 4 shows a second embodiment of a magnetocaloric device 1. Thesecond embodiment comprises a micro-magnetocaloric device 1 whichoperates as a power generator with regeneration of the magnetic energyand applies the regeneration of heat performed by the thermal switchprinciple. In this particular case, at least one magnetocaloric material5 is embodied between two thermal switch mechanisms, denoted by TDcoldand TDhot respectively. To each of these thermal switch mechanisms,respective extended surfaces are attached, denoted by EScold and EShot,respectively. The basic operation is the following. The magnetic fieldsource 2 may comprise at least one iron or soft ferromagnetic materialcore 3, at least one coil winding 4 or other means for electric currentflow, and at least one electric circuit for the control, regulation ofoperation and regeneration of the magnetic energy. When the magneticfield source 2 is switched on, the thermal switch TDhot is in operation,thus represents means for thermal transport from the extended surfaceEShot to the magnetocaloric material 5. Through the extended surfaceEShot, the working fluid flows in a direction from the heat source heatexchanger HHEX to the heat sink heat exchanger CHEX, wherein thepropulsion of the working fluid is provided by the pumping means 6 or byany other working fluid propulsion system. In this manner, the workingfluid from HHEX transfers heat to the extended surface EShot and to theTDhot. In the CHEX, the working fluid removes heat from the system.Before the magnetic field source 2 is switched off, the thermal switchmechanism TDhot is deactivated, thus preventing the thermal transportbetween the magnetocaloric material 5 and the extended surface EShot,wherein the deactivated thermal switch mechanism TDhot may represent anadiabatic barrier. When the magnetic field source is switched off, thethermal switch mechanism TDcold is activated, thus representing meansfor thermal transport from the magnetocaloric material 5 via the thermalswitch TDcold to the extended surface EScold. Through the extendedsurface EScold, the working fluid flows in a direction from the heatsink heat exchanger CHEX to the heat source heat exchanger HHEX, whereinthe propulsion of the working fluid is provided by the pumping means 6or by any other working fluid propulsion system. In this manner, themagnetocaloric material 5 transfers heat to the working fluid via theTDcold to the extended surface EScold. This cycle is then repeated. Theworking fluid may flow continuously or discontinuously, depending ondesired thermodynamic cycle. Furthermore, the working fluid flow, theoperation of thermal switch mechanisms and the operation of the magneticfield source 2 can be tuned in a manner that the micro-magnetocaloricdevice operates with different thermodynamic cycles.

In a third embodiment of this invention, the micro-magnetocaloric device1 as shown in FIG. 5 operates as a refrigerator or as a heat pump withregeneration of the magnetic energy and applies the regeneration of heatusing a multifunctional coating on at least one magnetocaloric material5. In this particular case, the magnetocaloric material 5 is coated withtwo multifunctional coatings, denoted by multifunctional surface MScoldand multifunctional surface MShot, respectively. Each of thesemultifunctional coatings may be attached to or may represent a part ofthe fluid flow channels provided by the extended surfaces EShot andEScold, respectively.

The basic operation of the micro-magnetocaloric device 1 according tothe third embodiment is the following. The magnetic field source 2 maycomprise at least one iron or soft ferromagnetic material core 3, atleast one coil winding 4 or other means for electric current flow, andat least one electric circuit for the control, regulation of operationand regeneration of the magnetic energy. When the magnetic field source2 is switched on, the coating comprising the multifunctional surfaceMShot is in operation, thus representing a means to influence thethermal or velocity boundary layer, or wetting of the fluid flow. Bythis, the heat transfer between the magnetocaloric material 5 and theworking fluid is enhanced. The coating comprising the multifunctionalsurface MShot thus represents a means for influencing the thermaltransport from the magnetocaloric material 5 to the working fluid in theextended surface EShot. Through the extended surface Shot, the workingfluid flows in a direction from the heat source heat exchanger CHEX tothe heat sink heat exchanger HHEX, wherein the propulsion of the workingfluid is provided by the pumping means 6 or by any other working fluidpropulsion system. The working fluid absorbs heat from the extendedsurface EShot and the multifunctional surface MShot and rejects it inthe HHEX. Before the magnetic field source 2 is switched off, themultifunctional surface MShot is deactivated, thus representing a meansto prevent the thermal transport between the magnetocaloric material 5and the working fluid in the extended surface EShot. The deactivatedmultifunctional surface MShot may represent an adiabatic barrier byinfluencing the thermal and velocity boundary layer or wetting of theworking fluid. When the magnetic field source 2 is switched off, themultifunctional surface MScold is activated, thus representing a meansfor thermal transport from the working fluid in the extended surfaceEScold to the magnetocaloric material 5, by influencing the thermal andvelocity boundary layer or wetting of the working fluid. Through theextended surface EScold, the working fluid flows in a direction from theheat sink heat exchanger HHEX to the heat source heat exchanger CHEX,wherein the propulsion of the working fluid is provided by the pumpingmeans 6 or by any other working fluid propulsion system. In this manner,the working fluid transfers heat from the extended surface EScold andthe multifunctional surface MScold to the magnetocaloric material 5.This cycle is then repeated. The working fluid may flow continuously ordiscontinuously, depending on desired thermodynamic cycle. Furthermore,the working fluid flow, the operation of multifunctional coatings andthe operation of the magnetic field source 2 can be tuned in a mannerthat the micro-magnetocaloric device operates with differentthermodynamic cycles.

In the fourth embodiment of this invention, as shown in FIG. 6, themicro-magnetocaloric device 1 operates as a power generator withregeneration of the magnetic energy and applies the regeneration of heatperformed by multifunctional coatings on the magnetocaloric material 5.In this particular case, the magnetocaloric material 5 is coated withtwo multifunctional coatings, denoted by multifunctional surface MScoldand multifunctional surface MShot, respectively.

Each of these coatings are attached to or represent a part of the fluidflow channels provided by extended surfaces, denoted by extended surfaceEScold and extended surface EShot, respectively. The basic operation ofthe micro-magnetocaloric device 1 according to the fourth embodiment isthe following. The magnetic field source 2 comprises at least one ironor soft ferromagnetic material core 3, at least one coil winding 4 orother means for electric current flow, and at least one electric circuitfor the control, regulation of operation and regeneration of themagnetic energy. When the magnetic field source 2 is switched on, thecoating comprising the multifunctional surface MShot is in operation,thus representing a means to influence the thermal or velocity boundarylayer, or wetting of the fluid flow. By this, the heat transfer betweenthe magnetocaloric material 5 and the working fluid is enhanced. Themultifunctional surface MShot thus represents a means for thermaltransport from the working fluid to the magnetocaloric material 5.Through the extended surface EShot, the working fluid flows in adirection from the heat source heat exchanger HHEX to the heat sink heatexchanger CHEX, wherein the propulsion of the working fluid is providedby the pumping means 6 or by any other working fluid propulsion system.In this manner, the working fluid transfers heat to the extended surfaceEShot and the multifunctional surface MShot. In the heat sink heatexchanger CHEX, the working fluid rejects heat out of the system. Beforethe magnetic field source 2 is switched off, the multifunctional surfaceMShot is deactivated, thus representing a means to prevent the thermaltransport between the magnetocaloric material and the working fluid inthe extended surface EShot by influencing the thermal and velocityboundary layer or wetting of the working fluid. Therein, the deactivatedmultifunctional surface MShot may represent an adiabatic barrier. Whenthe magnetic field source 2 is switched off, the multifunctional surfaceMScold is activated, thus representing a means for thermal transportfrom the magnetocaloric material 5 via the extended surface EScold andthe multifunctional surface MScold to the working fluid, by influencingthe thermal and velocity boundary layer or wetting of the working fluid.Through the extended surface EScold, the working fluid flows in adirection from the heat sink heat exchanger CHEX to the heat source heatexchanger HHEX, wherein the propulsion of the working fluid is providedby the pumping means 6 or by any other working fluid propulsion system.In this manner, the magnetocaloric material 5 transfers heat to theworking fluid via the extended surface EScold and the multifunctionalsurface MScold. This cycle is then repeated. The working fluid may flowcontinuously or discontinuously, depending on desired thermodynamiccycle. Furthermore, the working fluid flow, the operation ofmultifunctional coatings and the operation of the magnetic field source2 can be tuned in a manner that the micro-magnetocaloric device operateswith different thermodynamic cycles.

FIG. 7 shows a fifth embodiment, wherein several magnetocaloricmaterials 5 and an especially designed electric resistive multipolemagnetic field source 2 with the regeneration of magnetic energy form amagnetocaloric refrigerator or a heat pump. The principle of theoperation of the fifth embodiment is similar as described above for thefirst embodiment.

FIG. 8 shows a sixth embodiment, wherein several magnetocaloricmaterials 5 and an especially designed electric resistive multipolemagnetic field source 2 with the regeneration of magnetic energy form amagnetocaloric power generator. The principle of the operation of thesixth embodiment is similar as described above for the secondembodiment.

FIG. 9 shows a seventh embodiment, wherein the micro-magnetocaloricdevice 1 operates as a refrigerator or as a heat pump with regenerationof the magnetic energy and applies the regeneration of heat performed bythe multifunctional coating on the magnetocaloric material 5. This kindof solution comprises several magnetocaloric materials 5 and anespecially designed electric resistive multipole magnetic field source2. In this particular case, the magnetocaloric material 5 is coated withtwo multifunctional coatings, denoted by multifunctional surface MScoldand multifunctional surface MShot respectively. The principle of theoperation of the seventh embodiment is similar as described above forthe third embodiment.

FIG. 10 shows an eighth embodiment, wherein the micro-magnetocaloricdevice 1 operates as a power generator with regeneration of the magneticenergy and with the regeneration of heat performed by themultifunctional coating on the magnetocaloric material 5. This kind ofsolution comprises several magnetocaloric materials 5 and an especiallydesigned electric resistive multipole magnetic field source 2. In thisparticular case, the magnetocaloric material 5 is coated with twomultifunctional coatings, denoted by multifunctional surface MScold andmultifunctional surface MShot respectively. The principle of theoperation of the eighth embodiment is similar as described above for thefourth embodiment.

In the ninth embodiment as shown in FIG. 11, an alternative mechanismfor the heat transfer between the magnetocaloric material 5 and theworking fluid is given. This mechanism may be applied to any otherembodiment of the magnetocaloric device 1. This particular mechanism mayalso be combined with the thermal switch mechanisms or with themultifunctional coatings. The principle of the operation is thefollowing. For each magnetocaloric material 5, the extended surface withthe fluid channel(s) is attached above and below the magnetocaloricmaterial. In the embodiment shown in FIG. 11, a pair of magnetocaloricmaterials 5 is provided and two different operation modes of the deviceare shown on the left and right sides of FIG. 11, respectively. Inoperation mode 1 as shown on the left side in the FIG. 11, the uppermagnetocaloric material 5′ is magnetized, and the lower magnetocaloricmaterial 5″ is demagnetized. Since the magnetized MC material 5′ iswarm, the working fluid flows through the extended surface EShot fromthe CHEX to the HHEX. The extended surface EShot is in this casepositioned above the magnetized magnetocaloric material 5′. Below themagnetized magnetocaloric material 5′ is the extended surface EScold. Inthis case, the working fluid in the channels of the extended surfaceEScold is at rest. Because of the difference in the heat transfercoefficient between moving fluid and static fluid, most of the heat istransferred from the magnetized MC material 5′ to the working fluidwhich flows in the extended surface EShot. At the same time, the lowermagnetocaloric material 5″ is demagnetized. Therefore, the fluid flowsthrough the lower extended surface EScold which is in contact with thedemagnetized MC material 5″, while the fluid in the lower extendedsurface EShot stays at rest. In operation mode 2 as shown on the rightside in FIG. 11, the upper magnetocaloric material 5′ is demagnetized,and the lower magnetocaloric material 5″ is magnetized. Since thedemagnetized MC material 5′ is cold, the working fluid flows through theextended surface EScold below the demagnetized magnetocaloric material5′from the HHEX to the CHEX. Above the demagnetized magnetocaloricmaterial 5′ is the extended surface EShot. In this case, the workingfluid in the channels of the extended surface EShot is at rest. Becauseof the difference in the heat transfer coefficient between moving fluidand static fluid, most of the heat is transferred from the working fluid(which flows in the extended surface EScold) to the demagnetizedmaterial 5′. At the same time, the lower magnetocaloric material 5″ ismagnetized. Therefore, the fluid flows through the upper extendedsurface EShot while the fluid in the upper extended surface EScold staysat rest. The working fluid flow as shown for both operating modes inFIG. 11 may be continuous while the device is switched between the twooperation modes, wherein the working fluid flows in at least twoseparate channels as described above.

A tenth embodiment, as shown in FIG. 12, relates to a multipole magneticfield source 2 with several magnetocaloric materials 5 and multiplefluid channels which can be implemented in a cylindrical structure. Thisembodiment may comprise thermal switches, or it may comprisemultifunctional material coated on the magnetocaloric material 5 asdescribed above. The magnetocaloric device according to the tenthembodiment may be operated as a refrigerator, heat pump or powergenerator.

The various embodiments illustrated in FIGS. 3 to 12 have the feature incommon that the magnetocaloric material is positioned inside the gapbetween the magnet's poles, and the thermal switches or multifunctionalcoatings are positioned outside this gap, i.e. on the “top and bottom”surfaces of the magnetocaloric material, when the poles of the magnetare adjacent the side surfaces of the magnetocaloric material. Thisholds also for solutions, when a special method for the heat transfer isapplied as presented in FIG. 11.

Further alternative embodiments are illustrated in FIG. 13-17.

According to the first alternative embodiment as shown in FIG. 13, thelayered structure comprising the magnetocaloric material 5 and thethermal switches TDhot and TDcold, similar to the structure as describedabove in conjunction with FIG. 4, is positioned such that the respectivethermal switches TDhot and TDcold are located adjacent the poles of themagnet's iron core 3, with the magnetocaloric material 5 sandwichedbetween the thermal switches TDhot and TDcold. Thus, it can be ensuredthat the surfaces of the magnetocaloric material 5 which are in directcontact with the thermal switches TDhot and TDcold are located in aregion of maximum magnetic field strength.

FIG. 14 shows a second alternative embodiment, which differs from thefirst alternative embodiment of FIG. 13 in that multifunctional surfacesare provided instead of thermal switches.

In the third alternative embodiment, as shown in FIG. 15, thermalswitches and multiple magnetocaloric materials 5 are provided inconjunction with a magnetic field source 2 with several poles and in thefourth alternative embodiment, as shown in FIG. 16, multifunctionalsurfaces and multiple magnetocaloric materials 5 are provided inconjunction with a magnetic field source 2 with several poles.

FIG. 17 shows a fifth alternative embodiment, wherein a multipolemagnetic field source 2 is provided in conjunction with severalmagnetocaloric materials 5 and multiple fluid channels in a cylindricalstructure.

In all of the embodiments of the present invention, an electric circuit,as a part of the magnetic field source, may enable the regeneration ofthe magnetic energy. Embodiments of electric circuits are shown in FIGS.18, 20 and 21.

FIG. 18 shows a general schematic diagram of a circuit as part of themagnetic field source 2 for storing and regeneration of the magneticfield energy. FIG. 19 shows the schematics of the operation of a heatengine, based on the magnetocaloric effect and a Carnot thermodynamiccycle.

The energy of the magnetic field of the electromagnet can beequivalently expressed as

E_(magn)=½LI²,

where L is the inductance of the electromagnet and I is the current thatflows through the coil of the electromagnet. As shown in FIG. 18,switcher S1 is used to redirect the current that flows through theelectromagnet 3, 4 into an energy collector device 7, which could be abattery or a capacitor or other similar element. When the current stopsflowing, the magnetic field in the electromagnet vanishes and the energycollector device is charged.

To turn on the magnetic field in the electromagnet, the switch S2 isused to release the energy stored in the energy collector to start thecurrent flow through the electromagnet.

Due to resistive losses and losses of the energy collector device, avoltage source V is required, which maintains the current, once thecurrent flow is established.

The circuit primarily acts as an efficient switching and energy storagedevice. The operation, combined with magnetocaloric material, is thefollowing. In the case of refrigeration or heat pumping, the followingsteps are performed. When the magnetic field turns on (S2 turns on), themagnetocaloric material heats up, then heat transfer between themagnetocaloric material and working fluid is established (via advancedheat transfer solutions presented in this invention) so themagnetocaloric material dissipates heat into the working fluid. When themagnetic field is turned off (S1 turns on), the magnetocaloric materialcools and when brought in contact with the working fluid (via advancedheat transfer solutions presented in this invention), it absorbs heatfrom the working fluid.

When the magnetocaloric device 1 operates as a heat engine (powergenerator), the procedure is reversed. The operation for the Carnotthermodynamic cycle will be described, however one should note that alsoother thermodynamic cycles can be developed. In the first stage (FIGS.18, 19, processes 1 to 3) the switch S1 is turned on and the currentfrom the electromagnet starts charging the energy collector device.During this stage the magnetocaloric material would start cooling due tothe magnetocaloric effect, however when brought in contact with the hotworking fluid (hot thermal switches turned ON or multifunctional coatingON), the magnetocaloric material absorbs heat from the working fluid.Therefore, according to the example, the material is demagnetizingisothermally (FIG. 19, process 1-2). Then (FIG. 19, 2-3) the hot thermalswitches (TDhot) are turned off (or the multifuntional coating is setOFF) and the magnetocaloric material 5 is now thermally isolated,however still demagnetizing. Due to the magnetocaloric effect, themagnetocaloric material cools down. In the second stage (FIG. 19,processes 3 to 1), the switch S2 is turned on and the energy from theenergy collector device starts the current flow in the electromagnet.During this stage, the magnetocaloric material 5 would heat up due tothe magnetocaloric effect, however when brought in contact with theworking fluid (cold thermal switches TDcold are turned ON or themutifunctional coating on the cold side is set ON), the magnetocaloricmaterial 5 transfers heat into the working fluid. As the example, themagnetocaloric material 5 is magnetizing isothermally in this step (FIG.19, process 3-4). Then (FIG. 19, process 4-1) the cold thermal switchesTDcold are turned OFF (or the multifunctional coating is set OFF),therefore the magnetocaloric material 5 heats up. From thethermodynamics of the magnetocaloric effect, it follows that the energystored in the first stage of the processes (1 to 3) is larger then theenergy used in the second stage of the processes (3 to 4), that is,there remains a part of the energy that can be used for other mechanicalor electrical work.

FIG. 20 shows an implementation of the concept illustrated in FIG. 18.The energy collector device 7 in this case is a capacitor (condenser).The switching operation is explained below:

-   -   1. The current flows trough S1 and S2, from voltage source V to        ground GND.    -   2. S1 switches from GND to capacitor 7, now the current flows        into the capacitor 7.    -   3. At the moment when the current stops flowing, the capacitor 7        is charged and S1 and S2 are switched to neutral position.    -   4. Switching S1 to GND and S2 to the electromagnet 3, 4 starts        the current flow in the electromagnet 3, 4 (by discharging the        capacitor 7).    -   5. When the capacitor 7 is discharged (or when the desired        current is reached) S2 connects the electromagnet 3, 4 to the        voltage source to maintain the current in the electromagnet 3,        4.    -   6. The cycle is repeated.

FIG. 21 shows a further implementation of the circuit described in FIGS.18 and 20. It uses mosfets as switches (gate drivers are not shown). Theswitching operation is explained below:

-   -   1. The current flows in the electromagnet through diode D1 and        mosfet Q1, from voltage source V to ground GND.    -   2. Mosfets Q1 and Q2 are simultaneously turned off, which forces        the current in the electromagnet to charge the capacitor. When        the current stops flowing, the capacitor is charged and the        magnetic field vanishes.    -   3. Now mosfets Q1 and Q2 are simultaneously turned on. The        currents start flowing from the capacitor to GND. Diode D2        prevents the capacitor to discharge directly to GND and diode D1        prevents the current to flow from the capacitor into the voltage        source.    -   4. When the capacitor is dicharged the voltage source V        maintains the current in the electromagnet.    -   5. The cycle is repeated.

When the steps 1., 2., 3., 4. are carried out, the current in theelectromagnet changes in time as depicted in FIG. 22, which shows theflow of current through the electromagnet with respect to time, when thesteps 1.-4. are implemented in circuit shown on FIG. 21.

For all embodiments of the invention, the magnetic field source 2 withthe regeneration of magnetic energy, as described above, may alsorepresent a structure in which the magnetocaloric material 5 isembedded. Some embodiments thereof are shown in FIGS. 23 to 28. Notethat in all the cases of FIGS. 23 to 28, the heat transfer principlesdescribed in conjunction with the embodiment illustrated in FIGS. 3 to16 may be applied.

FIG. 23 shows a first embodiment of a magnetic field source 2 as astructure in which the magnetocaloric material 5 is embedded. Therein,the electric coil 4 of the magnetic field source 2 may surround themagnetocaloric material 5. In this particular case, the electric coil isin direct contact with a working fluid, or a multifunctional coating, ora thermal switch.

FIG. 24 shows a second embodiment of a magnetic field source 2 whereinthe magnetocaloric material 5 is embedded. Therein, a layer of softferromagnetic material 8 may be provided between the magnetocaloricmaterial 5 and the surrounding electric coil 4. Also in this particularcase, the electric coil is in direct contact with a working fluid, or amultifunctional coating, or a thermal switch.

FIG. 25 shows a third embodiment of a magnetic field source 2 whereinthe magnetocaloric material 5 is embedded. Therein, the magnetocaloricmaterial 5 is provided inside the iron core 3, wherein respectiveregions of a soft ferromagnetic material 8, which are surrounded byrespective section of electric coil 4, are located at opposite sides ofthe magnetocaloric material 5. In contrary to examples in FIGS. 23 and24, in this particular case, the magnetocaloric material is coated orembodied as for other examples within this invention.

FIG. 26 shows a fourth embodiment of a magnetic field source 2 whereinthe magnetocaloric material 5 is embedded. In contrast to the thirdembodiment of FIG. 25, the magnetocaloric material 5 abuts not only theadjacent regions of soft ferromagnetic material 8, but is also in directcontact with the electric coil 4 which surrounds the soft ferromagneticmaterial 8. In contrary to examples in FIGS. 23 and 24, also in thisparticular case, the magnetocaloric material is coated or embodied asfor other examples within this invention.

FIG. 27 shows a fifth embodiment of a magnetic field source 2 whereinthe magnetocaloric material 5 is embedded. Therein, the magnetocaloricmaterial is sandwiched between two layers of soft ferromagnetic material8. In contrary to examples in FIGS. 23 and 24, also in this particularcase, the magnetocaloric material is coated or embodied as for otherexamples within this invention.

FIG. 28 shows a sixth embodiment of a magnetic field source 2 whereinthe magnetocaloric material 5 is embedded. Therein, a multilayerstructure is formed, wherein layers of magnetocaloric material 5 areprovided with fluid channels 9 within the magnetic field source 2.Thermally conductive material 10 may be provided which may connect twoseparate layers of magnetocaloric material 5. The fluid channels 9 maybe formed, as described above, in and/or adjacent to thermal switches ormultifunctional coatings provided on respective surfaces of themagnetocaloric material.

The effects of applying the alternative embodiments shown in FIG. 23-28,compared to other embodiments described herein, is in the furtherminiaturization of the magnetic field source, and as the consequence, ofthe device. Furthermore, such embodiments can bring advantages incompensating for poor mechanical properties of some magnetocaloricmaterials. Moreover, these embodiments can also provide enhancement ofthe effective thermal conductivity. Moreover, but this holds also toother embodiments descride above, FIGS. 23-28 show modifications inwhich the magnetic field source magnetizes only the magnetocaloricmaterial 5 and not the empty space between the magnetocaloric materialand fluid voids. This last is actually a drawback of the well knownactive magnetic regenerator.

As an alternative to any of given solutions in this invention, themagnetic field source, besides of an electric coil, may comprise anadditional permanent magnet material.

1. Magnetocaloric device (1), comprising: at least one magnetocaloricmaterial (5) embedded between two heat transfer structures (TD_(hot),TD_(cold); MS_(hot), MS_(cold)); at least one electric source (2) forgenerating a magnetic field; and at least one hydraulic circuit in whichthe working fluid flows in a constant direction and which comprises atleast one propulsion means (6) for the working fluid, wherein the heattransfer structures (TD_(hot), TD_(cold); MS_(hot), MS_(cold)) areadapted to control the transfer or transport of heat between themagnetocaloric material (5) and the working fluid.
 2. The device (1)according to claim 1, wherein the heat transfer structures comprise atleast one thermal switch (TD_(hot), TD_(cold)) which is adapted tocontrol heat transfer or heat transport from the magnetocaloric material(5) to the hydraulic circuit and/or from the hydraulic circuit to themagnetocaloric material (5).
 3. The device (1) according to claim 1,wherein the heat transfer structures comprise at least onemultifunctional coating (MS_(hot), MS_(cold)) which is adapted to affectthe wetting effect of the working fluid, or/and to affect the thermal orvelocity boundary layer of the working fluid, or/and to affect thechemical protection of the magnetocaloric material (5), and/or to affectthe mechanical properties of the magnetocaloric material (5), and/or toaffect the effective thermal properties of the magnetocaloric material(5) and/or the multifunctional coating (MS_(hot), MS_(cold)).
 4. Thedevice (1) according to claim 1, wherein the electric source (2)comprises electric windings (4), a core (3) for the manipulation of themagnetic flux direction, and an electric circuit which enablesregeneration of magnetic energy.
 5. The device (1) according to claim 1,where the hydraulic circuit with the working fluid is connected to atleast one heat exchanger (CHEX, HHEX), such as a heat source or/and aheat sink heat exchanger.
 6. The device (1) according to claim 1,comprising at least one magnetic field source and a plurality ofmagnetocaloric materials (5), wherein each magnetocaloric material (5)is embedded between two heat transfer structures (TD_(hot), TD_(cold);MS_(hot), MS_(cold)), and wherein a common hydraulic circuit is providedsuch that the heat transfer structures (TD_(hot), TD_(cold); MS_(hot),MS_(cold)) are adapted to control the transfer or transport of heatbetween each magnetocaloric material (5) and the working fluid.
 7. Thedevice (1) according to claim 1, wherein several sub-devices form acascade system.
 8. The device (1) according to claim 2, wherein thethermal switch (TD_(hot), TD_(cold)) comprises at least one thermalswitch material which exhibits anisotropy of the thermal conductivity orcomprises at least one thermal switch composite material which exhibitsanisotropy of the effective thermal conductivity.
 9. The device (1)according to claim 2, wherein the thermal switch (TD_(hot), TD_(cold))is based on mechanical contact by elastomer materials, or liquidcrystals, or based on ferrofluids, or magnetorheologic principles, orliquid metals, or electrorheologic principles, or electrowettingprinciples, or electrophoresis principles, or magnetohydrodynamics, orelectrohydrodynamics.
 10. The device (1) according to claim 2, whereinthe thermal switch (TD_(hot), TD_(cold)) is based on thermoelectric(Peltier or Seebeck), or thermionic, or spincaloritronic (spin Peltieror spin Seebeck) transport effects.
 11. Magnetocaloric device (1),comprising: at least one magnetocaloric material (5) embedded betweentwo heat transfer structures (TD_(hot), TD_(cold); MS_(hot), MS_(cold));at least one electric source (2) for generating a magnetic field whichenables regeneration of the magnetic energy; and at least one hydrauliccircuit which comprises at least one propulsion means (6) for theworking fluid, wherein the heat transfer structures (TD_(hot),TD_(cold); MS_(hot), MS_(cold)) are adapted to control the transfer ortransport of heat between the magnetocaloric material (5) and theworking fluid.
 12. Magnetocaloric device (1) according to claim 11,wherein the electric source (2) comprises an electromagnet (3, 4) and anenergy collector device (7), wherein the electric source (2) is adaptedto charge the energy collector device (7) when the magnetic field of theelectromagnet (3, 4) is turned off, and use the charged energy collectordevice (7) for generation of a magnetic field in the electromagnet (3,4) when the magnetic field is turned on.
 13. Magnetocaloric device (1)according to claim 12, wherein the electric source (2) further comprisesa first switching device (S1) for connecting the electromagnet (3, 4) tothe energy collector device for charging the energy colletor device (7)and a second switching device (S2) for connecting the energy collectordevice (7) to the electromagnet (3, 4) for turning on the magnetic fieldin the electromagnet by releasing the energy stored in the energycollector device (7) to start the current flow through the electromagnet(3, 4).
 14. Magnetocaloric device (1) according to claim 11, wherein theenergy collector device (7) comprises a battery or a capacitor. 15.Magnetocaloric device (1) according to claim 1, wherein the magneticfield is generated by at least one electric source (2) and at least onepermanent magnet material, and where the electric circuit enablesregeneration of the magnetic energy.